Battery chargers generally fall into two categories--(1) direct current (D.C.) chargers and (2) pulsed current chargers.
Direct current chargers typically utilize either a constant voltage mode in which the voltage is fixed and the current varies, or a constant current mode in which the current is fixed and the voltage varies. D.C. chargers give rise to several problems, many of which can be reduced or eliminated by limiting the maximum charging current to a low-value, and extending the charge cycle up to several hours. A typical low-value charging current would be one-tenth battery capacity, i.e., where the charging current falls at the battery's nominal amp-hour capacity divided by 10 hours. Thus, a ten amp-hour battery charging at a rate of 1 amp would employ a low-value charging current. Such chargers, known as trickle chargers, are advantageous in that they obviate the need for complex control schemes, and minimize the danger of reaching an overcharge condition. This is especially true in the constant voltage mode since current will reduce even further as battery voltage approaches the voltage of the charging source. The main drawback of trickle chargers is the inconvenience of being unable to use the battery for the 8 to 18 hours that are typically required to recharge, or alternatively, the expense of procuring additional battery packs to act as replacements during the recharge cycle. These disadvantages are especially relevant with respect to electric vehicle such as golf carts in which the batteries form an integral part of the device, or are relatively large and difficult to handle.
Trickle chargers, along with other D.C. chargers, are also problematic in that they tend to cause chemical breakdown (electrolysis) of the electrolyte. The phenomenon is common to all forms of rechargeable batteries, but is most commonly recognized in lead-acid batteries. In electrolysis, gasses form a boundary layer at the electrodes and interfere with the recharging process. The build-up of gasses increases the apparent impedance of the battery and causes current related heating that may result in failure of internal structures, or in the most severe case, an explosion. Even without damage or danger of explosion, the gasses may require venting and are generally hazardous. Electrolysis may also cause loss of electrolyte which is deleterious to the battery chemistry, causing reduced battery life and increased maintenance costs.
In pulsed battery chargers, the charging current is turned on and off periodically, thus allowing the gasses sufficient time to recombine into the electrolyte solution. A further improvement can be achieved by utilizing the period of recombination to apply short discharge pulses to the battery to "clean-up" the newly plated material, thereby eliminating contaminants and nodules in the plated matrix. This technique was originally developed and patented by G. W. Jernstedt (assigned to Westinghouse Electric) between 1948 and 1954, and adapted to battery chargers by W. B. Burkett and others (assigned to Christie Electric Corp) around 1971.
An added benefit of pulsed charging is that it allows much higher current density in the charge pulse, which may significantly reduce the charge time. There are practical considerations such as current carrying capacity of the internal battery structure that must be observed, so extremely short charge cycles (less than 0.1 hour) are rarely practical, but still may be possible. Major concern of a high rate charging system centers around when to stop charging, since even a moderate overcharge will cause battery temperature to rise drastically, and can cause explosion. Traditional approaches have been to stay on the safe side and terminate the charge before peak capacity has been achieved. More complex control schemes have been devised (e.g. U.S. Pat. No. 4,746,852 to Martin), but are largely limited to specific battery types where the charge curve is predictable. Many of these approaches depend on further instrumentation of the battery pack through addition of temperature sensors. In the case of the example above, identification modules are used to select a specific control mode based upon the signaling of a specific battery type. As used herein, battery type refers to the energy storage chemistry used in the battery. Popular battery types include lead-acid, nickel-cadmium, and nickel-metal-hydride chemistries.
Where multiple batteries are to be charged, there may be a significant savings in using a single charger to charge more than one battery. Concurrent charging of multiple batteries using a single charger is already known, as exemplified by common household rechargers for AAA, AA, C and D cell batteries, and as set forth in U.S. Pat. No. 4,237,409 to Sugalski (Dec. 2, 1980).
Automated sequential charging of multiple batteries using a single charger is also known, but only for ex situ batteries, i.e., batteries which are not installed in end-user equipment being powered by the batteries. U.S. Pat. No. 5,206,577 to Fish (Apr. 27, 1993), for example, describes charging of AA and other cylindrically shaped batteries by moving the batteries sequentially through discharging and charging positions under the influence of gravity or some other biasing agent. Similarly, U.S. Pat. No. 5,357,187 to Park (Oct. 18, 1994) discloses sequential charging of batteries under control of a microprocessor. The closest art uncovered to date regarding sequential in situ charging of batteries is U.S. Pat. No. 5,028,259 to Johnson et al. (Jul. 2, 1991), which describes sequential charging of two telephone batteries, one, but not both of which can be charged in situ.
Thus, despite advancements in pulsed charging, and the consequent reduction of charging times to the point that even a plurality of batteries can be realistically charged in a sequential fashion using a single charger, the automated sequential charging of in situ batteries using a single charger has escaped recognition. Thus, there is a need to provide a battery charger which can sequentially charge a plurality of batteries in situ, especially where the batteries are large, i.e., not hand held.